Background Field
Embodiments of the subject matter described herein are related generally to optical metrology, and more particularly to optical metrology using a model based approach.
Relevant Background
Semiconductor and other similar industries, often use optical metrology equipment to provide non-contact evaluation of substrates during processing. Optical metrology techniques, such as ellipsometry and reflectometry, typically operate by illuminating a sample with a probe beam of electromagnetic radiation and then detecting and analyzing the reflected and/or transmitted energy. The probe beam may be polarized or unpolarized radiation, and may include one or more wavelengths of radiation. Ellipsometry typically measures changes in the polarization state of the reflected beam after interacting with the sample, while reflectometry measures changes in the magnitude of the reflected beam.
One type of optical metrology utilizes a model based approach to determining parameters of interest in a sample under test. For example, spectroscopic optical critical dimension (OCD) determines parameters such as critical dimension (CD), sidewall angle (SWA) and thicknesses of features of a target by fitting spectra simulated using a model to experimental spectra from the sample under test. The fitting process may use a cost function in which the model parameters, i.e., the CD's, SWA's, thicknesses, etc, are varied until a best fit is obtained or a library may be used.
It is desirable to remove sources of error in optical metrology techniques to reduce or eliminated contributors of the total measurement uncertainty (TMU).